The invention relates to an automotive electrical alternator, and particularly to an alternator having a rotor assembly of a hybrid design including permanent magnets mounted thereon.
This invention is related to an electrical alternator, particularly adapted for use in motor vehicle applications including passenger cars and light trucks. These devices are typically mechanically driven using a drive belt wrapped on a pulley connected to the crankshaft of the vehicle""s internal combustion engine. The belt drives a pulley on the alternator which rotates an internal rotor assembly to generate alternating current (AC) electrical power. This alternating current electrical power is rectified to direct current (DC) and supplied to the motor vehicle""s electrical bus and storage battery.
While alternators have been in use in motor vehicles for many decades, today""s demands on motor vehicle design, cost, and performance have placed increasing emphasis on the design of more efficient alternators. Today""s motor vehicles feature a dramatic increase in the number of electrical on-board systems and accessories. Such electrical devices include interior and exterior lighting, climate control systems; increasingly sophisticated power train control systems, vehicle stability systems, traction control systems, and anti-lock brake systems. Vehicle audio and telematics systems place further demands on the vehicle""s electrical system. Still further challenges in terms of the output capacity of the motor vehicle""s electrical alternators will come with the widespread adoption of electrically assisted power steering and electric vehicle braking systems. Compounding these design challenges is the fact that the vehicle""s electrical system demands vary widely, irrespective of the engine operating speed which drives the alternator and changes through various driving conditions.
In addition to the challenges of providing high electrical output for the vehicle electrical alternator, further constraints include the desire to minimize the size of the alternator with respect to under hood packaging limitations, and its mass which relates to the vehicle""s fuel mileage.
In addition to the need of providing higher electrical output, designers of these devices further strive to provide high efficiency in the conversion of mechanical power delivered by the engine driven belt to electrical power output. Such efficiency translates directly into higher overall thermal efficiency of the motor vehicle and thus into fuel economy gains. And finally, as is the case with all components for mass-produced motor vehicles, cost remains a factor in the competitive offerings of such components to original equipment manufacturers.
One method which has been used to increase the power output of conventional electric alternators is to mount one or more permanent magnets onto an outer surface of the rotor to produce a significantly stronger magnetic field. The combined effect of the permanent magnets and the wound field coil enables the machine to produce substantially more electrical power output. It is difficult, however, to secure the permanent magnets onto the rotor because the rotor rotates at speeds of up to 25,000 RPM and is exposed to various environmental conditions.
One method of securing the magnets onto the rotor is to lengthen the intermeshing fingers of the pole pieces so that they extend outward over the magnets to hold the magnets onto the adjacent pole piece. This however adds stress to the fingers when the rotor is spinning rapidly which causes the fingers to flex outward into the gap between the rotor and the stator. The distance between the rotor and the stator is controlled very closely because the closer the rotor and stator are to one another, the higher the electrical power output will be. Flexation of the fingers outward will force the alternator to be designed with a larger gap which will reduce the power output of the alternator.
Further, the permanent magnets are generally brittle and require expensive grinding operations to meet size tolerances, thereby making it difficult and expensive to shape the permanent magnets in a manner that will allow the permanent magnets to be locked onto the rotor.
Therefore, there is a need for an alternator rotor assembly having permanent magnets mounted thereon to increase the power output of the alternator, wherein the permanent magnets are mounted onto the rotor in a way that does not affect the structural robustness of the rotor and does not require tight tolerances on the size and shape of the permanent magnets.
In a first aspect of the present invention, a rotor assembly includes first and second poles each having a plurality of pole fingers spaced radially about and extending axially from a periphery of the poles and a plurality of mounting surfaces spaced radially about and between the pole fingers. In the assembled condition, the pole pieces fit together such that the pole fingers interleave in the well known xe2x80x9cclawpolexe2x80x9d configuration. Permanent magnets are positioned on the mounting surfaces and threaded fasteners are provided to secure the permanent magnets to the pole pieces.
In another aspect of the present invention, each of the permanent magnets includes a plate mounted onto a top surface of the permanent magnet to protect the permanent magnet and to distribute the clamp force on the magnet that is applied by the fastener. The plate may also be used to help to spatially distribute the magnetic flux from the magnet.
In still another aspect of the present invention, each of the pole fingers of the first pole piece includes a distal end that extends between the pole fingers of the second pole piece. The distal ends of the pole fingers of the first pole piece are positioned over the mounting surfaces of the second pole piece. Each of the pole fingers of the second pole piece includes a distal end that extends between the pole fingers of the first pole piece. The distal ends of the pole fingers of the second pole piece are positioned over the mounting surfaces of the first pole piece. The fasteners extend through the distal ends and through the permanent magnets to secure the distal ends of the fingers, along with the permanent magnets, to the mounting surfaces.
In yet another aspect of the present invention the permanent magnets are secured to the plates with an adhesive and the permanent magnets are secured to the mounting surfaces with an adhesive.
In still another aspect of the present invention the plates are made from a magnetic material and the threaded fasteners are made from a non-magnetic material.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.